The present invention relates in general to electric power systems for electric vehicles, and, more specifically, to methods and apparatus for controlling DC/DC converters to convert a high voltage from a battery pack to a lower voltage for use on a low voltage bus in an electric vehicle.
Electrified vehicles such as battery electric vehicles and hybrid-electric vehicles typically utilize a high voltage power bus driven by a DC power source which may include storage and/or conversion devices such as a multi-cell battery pack. The battery pack may have a plurality of battery cells connected in series in order to provide the necessary power and/or voltage levels. The battery cells require real-time monitoring in order to maximize efficiency and performance, as well as to determine a battery state-of-charge (SOC) to predict a remaining driving range under battery power. Common battery types such as lithium ion (Li-Ion) use a large number of individual battery cells stacked together (connected in series and/or parallel), and groups of cells may be connected hierarchically in groups with monitoring of the groups rather than individual cells. As used herein, battery unit refers to an individual cell or a group of cells treated together.
In addition to the high voltage components associated with driving traction motors in the electrified vehicle, the vehicle also contains lower voltage electrical components and accessories (e.g., control modules, lighting, communications, and entertainment devices) as well as a lower voltage battery for supporting the low voltage components. In order to supply power from the main, high voltage battery pack to the low voltage components and/or to recharge the low voltage battery, a DC/DC converter has been used to down convert the high voltage to an appropriate lower voltage to drive a low voltage power bus.
Although it would be possible to tap into a small section of the battery pack to obtain the lower voltage, the resulting unbalanced drain of power from the battery pack would be undesirable. On the other hand, using a single DC/DC converter driven directly across the full high voltage of the battery pack requires high voltage components in the converter which results in a high cost. In order to obtain the necessary voltage conversion and to balance the electrical load among the plurality of battery cells, a bank of DC/DC converters has been used with the input of each converter connected to a different battery cell (or unit of cells) and with the converter outputs connected in parallel, as shown for example in U.S. Pat. No. 8,115,446 of Piccard et al, the disclosure of which is incorporated herein by reference.
A typical battery cell in the battery pack may generate about 4V. A target or setpoint voltage for the low voltage bus may be about 14V, for example. If each DC/DC converter covers one cell, then it is controlled to increase the voltage from 4V to 14V. If each converter covers six cells in series, then it is controlled to decrease the 24V across its input to the desired 14V.
U.S. patent application publication 2015/0214757A1 of Zane et al. discloses a plurality of DC/DC bypass converters with the outputs likewise connected in parallel, wherein operation of each converter is individually adjusted according to a battery state for its respective battery unit, to thereby decrease a rate of divergence of the battery state from a reference state. Thus, the states of charge for the battery units are more uniform, which improves overall performance of the battery pack. However, a side effect of independently varying the power from each DC/DC converter is that the common output voltage derived from the parallel connection of the converters may not remain constant at the desired value or range of values. A resulting voltage instability on the low voltage DC bus can be detrimental to component operation and to a rapid loss of life of the low voltage battery, especially since the total low voltage power loading may change rapidly during vehicle use.
In copending U.S. application Ser. No. 15/237,994, filed concurrently herewith, entitled “Electrified Vehicle Power Conversion for Low Voltage Bus,” which incorporated herein by reference in its entirety, a control strategy is disclosed wherein a first controller receives an actual bus voltage. The first controller generates a target total current in response to the bus voltage which is adapted to regulate the actual bus voltage to a target voltage. A second controller distributes the target current into a plurality of allocated current commands for respective converters according to respective states of charge of the battery units connected to the converters. As a result, power is drawn from the battery units in a way that balances their states of charge while a stabile voltage is maintained on the low-voltage bus.
Copending U.S. application Ser. No. 15/237,994 includes a distributed control architecture wherein a central control module determines both 1) the combined current flow to be obtained that regulates the common converter output voltage to the setpoint voltage, and 2) the distribution or allocation of that total current among the converters which results in the desired balancing of the battery unit states of charge. In the distributed architecture, the allocated current commands are transmitted to each DC/DC converter where a controller is responsible for regulating the output of the converter so that it achieves the allocated current. In order to maintain the desired output voltage without degrading the allocated current too much or for too long, this system must update the commanded current to each converter quite quickly. A fast response time is needed because output load demands on the low-voltage bus (e.g., power steering, headlights, radio, etc.) change very quickly and unpredictably since many can be switched on or off at any time by the vehicle occupants. Detailed analysis and simulations suggest that a maximum sample interval of 1 ms for updating the commands should be maintained. While achieving this update rate is possible, it would require more communication bandwidth than is available from currently available on-vehicle protocols (e.g., CAN, SPI, etc.). To speed up the communication, a dedicated communication bus could be employed, but would result in added cost and complexity.
It would be desirable to maintain output voltage control together with the cell balancing logic, without necessitating a large communication bandwidth or the associated extra costs.